Discriminator in a 246

[PhillyPhoto]

I was looking through the wiki and noticed the 246 can be modified for a discriminator tap. All that is listed is a site with photos (http://home.ica.net/~phoenix/wap/BC246T/), but does anyone have a more detailed explanation for the procedure? Also, what is this red thing used for, and what is it called??

 

[ka3jjz]

That 'red thing' is likely a capacitor, used to couple a signal and reduce loading on the circuit.

The explanation of how to do the tap hasn't changed very much from Bill Cheek's day (the one generally credited with starting the tap craze...). The instructions here are still valid, altho Bill uses some break apart pins. I'm quite certain only the actual point where the tap would need to be installed would be significantly different...

I wouldn't go fooling around with this surface mounted stuff unless you know what you are doing. There are others that install taps for a fee - I'd recommend that route, instead of trying to hack your way through it - otherwise you could wind up with a handheld paperweight. Even better, get an el-cheapo scanner (preferably 800 mhz capable, not necessary to be a trunker) at a local hamfest to modify. We're getting into that season very soon now... 73s Mike

 

[PhillyPhoto]

OK, I'll ask around to see if anyone can do it for me. I got the scanner for free, and since it's analog, I don't get too much use out of it now, so I figured I'd use if for trunker and what not when I'm not monitoring. Thanks.

 

[br0adband]

I just opened up my 246T for a tap job and was dismayed to see just how tiny that pin on the IC is. Guess I'm getting old an I'll need to get myself a magnifying glass to manage the soldering job. I'm not sure about that red thing in the pic, however: one wouldn't be using a capacitor in such a "capacity" (sorry, had to do it) since a resistor is what's called for to limit the signal.

So I'm guessing it's just a weird style resistor since I've never seen one like it before.

I'll have to do some thinking about whether or not I want to attempt this tap at this point in time. Most likely I'll make it happen but I definitely need to get a finer point tip for my lowly 25w soldering iron. I've been thinking about getting one of those "cold" soldering irons that heat up nearly instantly and cool off just as quick. I'd hate to damage this 246T for a tap job, that's for certain.

The soldering is easy, it's just the size of this circuit is so much smaller because of all the SMD stuff - and there's an SMD diode just to the southeast of that IC pin that needs to be tapped also, so a soldering iron tip "in the neighborhood" could have dire consequences if I don't keep my hand steady.

 

[radio50]

Also, what is this red thing used for, and what is it called??

I think that red thing is a 10 microfarad, 16 volt, tantalum capacitor (a type of polarized capacitor), with the + terminal connected to the chip pin. A capacitor can be used to block the flow of dc current through a circuit. Partway down this page they show such items:
http://www.piclist.com/images/www/hobby_elec/e_capa.htm

 

[PhillyPhoto]

I think that red thing is a 10 microfarad, 16 volt, tantalum capacitor (a type of polarized capacitor), with the + terminal connected to the chip pin. A capacitor can be used to block the flow of dc current through a circuit. Partway down this page they show such items:
http://www.piclist.com/images/www/hobby_elec/e_capa.htm

Thanks!

 

[slicerwizard]

I'm not sure about that red thing in the pic, however: one wouldn't be using a capacitor in such a "capacity" (sorry, had to do it) since a resistor is what's called for to limit the signal.

The images in that directory clearly show that the tap is being used to drive a two level data slicer. No series resistor is required to interface with a data slicer.

A small resistor (about 10k) can (and generally should) be added in series to isolate the discriminator circuit from the audio cable. The resistor also provides protection against a short circuit at or beyond the jack. It does not provide much in the way of useful signal limiting.

So I'm guessing it's just a weird style resistor since I've never seen one like it before.

It's a 10uF 16V tantalum capacitor. It serves as a DC blocker. Every discriminator tap should have a DC block installed. The large value (10uF) ensures that low frequency signals (like LTR and PassPort low speed data) will not be distorted.

and there's an SMD diode just to the southeast of that IC pin that needs to be tapped also

Excuse me? What for?